Because meat has higher nutrient density. Quit drinking the plant based kool-aid. Literally all of our health problems started in the 70's when they introduced the food pyramid telling people to reduce their meat and saturated fat intake and making breads and grains the entire base of the pyramid. Since the introduction of the food pyramid, obesity and diabetes exploded exponentially. Literally reducing meat intake and replacing fat with sugar for flavor is why we're in this health mess.

In addition, just because plants have a nutrient doesn't mean we absorb it well. There's a reason for example plant based iron is inferior to heme iron, our ability to absorb and use it is so much poorer. Humans are foregut digesters, not hindgut digesters. We have very small large intestines and we lack a cecum. Meanwhile we have extremely acidic stomachs (pH 1.5) and very long small intestines. This ultimately means humans are primed to eating meat, fruit, and tubers, in that order of prioritization. We evolved to eat foods we could absorb quickly and efficiently to provide the energy and nutrient density to fuel our power hungry brains.

Eating less meat isn't the answer at all, cutting out grains, sugar, and seed oils is the answer.